This is a continuing study of knee joint mechanics and its relationship to the treatment of osteoarthritis and total knee arthroplasty. The present application focuses on two topics: patello-femoral mechanics and the treatment of unicompartmental arthritis using a unicondylar prosthesis. The general approach to these studies continues to combine biomechanical functional evaluations of patients in the Gait Laboratory during activities of daily living with experimental and analytical models of the knee joint to assist in the analysis and an interpretation of the experimental results. We will test the hypothesis that changes in patello-femoral position between preoperative and postoperative patellar alignment will produce functional changes in patients following total knee replacement. These changes will produce extrinsic load changes on the knee joint and appear as altered patterns of moments sustained by the quadriceps mechanisms at specific angles of knee flexion. Patients will be placed into three groupings according to changes from preoperative alignment in superior-inferior alignment, medial-lateral and anterior-posterior directions. An in vitro cadaver model will be used to quantitate the intrinsic changes that result from changing the patellar alignment in a manner identical to those obtained from the radiographic groupings. Changes in patellar tracking, quadriceps to patellar tendon transfer ratios and retropatellar forces. A second hypothesis tests the relationship between preoperative extrinsic loads on the knee joint during activities of daily living and the long-term clinical outcome of unicompartmental knee replacement. The approach to this study is similar to that taken with the study of patients treated with high tibial osteotomy. The influence of joint laxity in the presence or absence of the ACL may play an important functional role that can be evaluated using both patient preoperative and postoperative testing as well as with simulation models to predict the distribution of load between lateral and medial compartments. Ultimately, this information will be useful in identifying functional loading characteristics that correlate with clinical results and radiographic changes.